In recent years, electronic equipment has been decreased in size, thickness, and weight and there has been a demand for wiring boards having refined wirings at high density by multilayering, refinement of wirings, and reduced thickness to be used for such electronic devices. Further, with an aim of decreasing environmental burdens, lead free solder has been used progressively in the relevant field. Correspondingly, improvement in heat resistance is demanded for wiring materials such as TAB (Tape Automated Bonding) tape, FPC (Flexible Printed Circuit), MFJ (Multi Frame Joiner), etc. An insulation layer for the wiring materials basically includes a substrate film and an adhesive layer. Such examples are shown in JP-A Hei 5 (1993)-29399, which describes heat resistant films, for example, of polyimide, polyether imide, polyphenylene sulfide, polyether ether ketone, etc. and organic insulation films comprising composite heat resistant films such as epoxy resin-glass cloth, epoxy resin-polyimide-glass cloth, etc. as the substrate film. It discloses an adhesive composition comprising a polyamide resin and an epoxy resin as the adhesive layer.
However, the adhesive composition of JP-A-Hei 5 (1993)-29399 involves a problem that a storage stability is low due to high reactivity between the amino group and the epoxy resin present in the polyamide resin structure. For overcoming the problem, JP-A-2004-136631 proposes an adhesive composition comprising a phenoxy resin having an epoxy group on both terminal ends, an acrylic rubber, and a curing agent. For the phenoxy resin, a bisphenol A type, a bisphenol F type, a bisphenol AD type, a bisphenol S type, and a copolymer type comprising bisphenol A and bisphenol F are exemplified. The adhesive composition of JP-A-2004-136631 involves problems that the adhesion strength is only about 0.5 kN/m although the phenoxy resin which is considered to have a relatively excellent adhesion strength is blended and that the soldering heat resistance is somewhat low as 260° C.
For overcoming the problems described above, JP-A-2010-150437 discloses an adhesive composition comprising a thermoplastic polyurethane resin having a weight-average molecular weight of 80,000 to 800,000, an epoxy resin, and an epoxy resin curing agent. While usual polyurethane resins involve a problem in the storage stability of an adhesive film due to high reactivity with an epoxy resin, the patent document describes that the storage stability can be improved by using a polyurethane resin having a molecular weight within a predetermined range. This resin composition has an adhesion strength of 1.1 to 1.7 kN/m. Further, JP-A-2010-143988 discloses that the soldering heat resistance of an adhesive composition comprising a polyurethane resin, an epoxy resin, and a novolac resin of a specific structure is 300° C. However, it has been generally known that polyurethane resins used in JP-A-2010-150437 and JP-A-2010-143988 are depolymerized at a temperature of 200° C. or higher. Since it is generally considered that the heat resistance of polyurethanes is 80 to 100° C., adhesives containing polyurethane resins involve concern about application use in the field of electronic equipment for industry and automobiles that require high heat resistance and high reliability. Further, the mechanism of suppressing the reaction with the epoxy resin by the molecular weight control of the polyurethane resin described in JP-A-2010-150437 is based on decrease in the concentration of functional groups but the reactivity per se is not changed, and no sufficient consideration has been taken for the effects of storing temperature or time.
JP-A-2009-67934 discloses a heat resistant self-fusing coating material containing a sulfonic group-containing polyhydroxy polyether resin and maleimide. However, the sulfonic group-containing polyhydroxy polyether resin described therein is different from a phenoxy resin having a bisphenol S skeleton used in the present invention. The problem of this heat resistant self-fusing coating material is that the fusing and curing temperature thereof is extremely high to issue a problem in view of processability.
The present invention intends to provide an adhesive composition excellent in storage stability, heat resistance, reliability, and adhesion property, and also capable of being adhered at low temperature, as well as an adhesive film and a wiring film using the adhesive composition.